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Engine Design Hits Turbulance

QANTAS knows small problems can lead to catastrophes.

THE dramatic crash of United Airlines flight 232 at Sioux City, Iowa, in 1989 remains one of the most famous examples of an aircraft brought down by uncontained engine failure.

The DC-10 suffered an uncontained failure of its No 2 engine, which damaged all three of the aircraft's hydraulic systems and rendered unworkable flight control surfaces used to steer and land the plane. The crew had to attempt the landing using only the thrust levers of the two surviving engines in an amazing feat of skill that came unstuck in the final approach when the right wing hit the runway. Television footage showed the aircraft apparently cartwheeling along the runway - an illusion created by the separation of the wing - as the main fuselage skidded sideways, rolled over and slid to a stop upside down in a cornfield.

The cause of the crash, which killed 111 of the 296 passengers, was later tracked down to a microscopic flaw introduced during the purifying of a titanium ingot used to make a fan disk, which propagated a tiny crack that engineers had failed to pick up. Airlines are aware that small things can cause mighty catastrophes. This is one reason Qantas is taking so much time to ensure it knows what happened to the Rolls-Royce Trent 900 engine that disintegrated on flight 32 near Singapore last week. Uncontained jet engine failures are rare and an analysis of accidents between 1998 and 2007 shows engine failures are at the root of just 2 per cent of fatal crashes.

Engines have also become more reliable: the in-flight shutdown rate has gone from 0.89 per 1000 hours for the piston-driven 1950s Boeing Stratocruiser to 0.0002 per 1000 hours for the Boeing 777-300ER.

"In the old days we used to pull an engine every 500 hours on the 747s," says University of NSW aviation expert Peter Marosszeky.

"The JT-9s at that stage were not as well designed mechanically and also the material technology wasn't there. Then the redesign of the engines altered all that. Now we have engines that literally stay on the wing for in excess of 40,000 hours, and that's a remarkable shift in the paradigm for these engines. They are very reliable."

All this, as Qantas chief executive Alan Joyce points out, makes the explosive failure of a relatively new Trent 900 on a two-year-old aircraft all the more mystifying.

Engineers have fingered the intermediate pressure turbine disc as the part that wreaked havoc with the engine after it disintegrated for unknown reasons. Containment devices made of kevlar (the substance used for bullet-proof vests) failed to stop parts ripping though a section of wing, destroying part of the engine cowling and even, according to a passenger, bouncing off the fuselage.

However, the containment ring may still have saved critical components of the aircraft from damage by flying debris that could have crippled the aircraft.

What caused the disc to disintegrate will likely be uncovered by Rolls-Royce engineers and Air Transport Safety Bureau sleuths, who have considerable expertise in metallurgical analysis, and parts recovered so far have been shipped to Britain for tests.

The Trent engines are a development of the RB211, the engine Rolls-Royce designed for jumbo jets and that drove the company into bankruptcy. More modern versions of this engine experienced an uncontained failure near San Francisco in August and shut down in Singapore this weekend, but these are viewed as coincidental events unrelated to the A380 problem. Part of the problem, says Marosszeky, is that Rolls-Royce opted for a more complicated three-shaft design rather than the simpler two-shaft design of competing engine-makers General Electric and Pratt & Whitney.

"Rolls-Royce engines over the years - and this goes back to when the first RB211 were flying - always did have an issue with the internals because they are a triple-spool engine, unlike the Pratts and unlike the GEs, and tolerances for operating and the balancing tolerances are super, super-sensitive in variations in oil pressure and temperature," he says.

Marosszeky also does not believe this is a case of faulty materials in the engine or their hi-tech nature. Rolls-Royce toyed with the idea of composite fan blades in RB211s but returned to titanium when these shattered when they had chicken carcasses fired at them to test for bird strike.

He says the components used in the Trent 900 are not lightweight per se but designed for their resistance to heat and stress.

"The issue as I see it, from the things I've seen and heard, is not so much the materials but in this particular case the way the engine was designed," he says.

"In other words, they seem to have problems with oil supply and keeping bearings and bearing shafts cool and free of heat."

Qantas says this is not the issue that has troubled it during the inspections, but it is worried about oil getting into places it should not on three of its engines.

Investigations of the engines is understood to have shown signs of spotting and pooling of oil in areas where it could catch fire.

According to respected industry journal Aviation Week, an August failure of a Trent 1000 at a test site in England was the result of an oil fire at high power. The Trent 1000 is a derivative of the Trent 900 and the heating is believed to have softened the intermediate pressure shaft, allowing the intermediate pressure turbine to spin too fast and disintegrate.

The issue caused consternation at Boeing because the engines are destined to power many of its new 787 Dreamliners and was another hiccup for the already delayed multi-billion-dollar program.

At least three engines have been taken off the A380s to further investigate the oil issue and Joyce says extensive checks are under way to explain "where oil shouldn't be on the engines".

"These are new engines on new aircraft and they shouldn't have these issues at this stage and so it's given us an indication of an area for us to focus into," he says.

Joyce rejects union attempts to claim maintenance issues are involved in the explosion, noting the engines have been maintained by Rolls-Royce since installation and the likely explanation is a materials or design issue.

The airline is also zeroing on the operational performance of the engines after none of the problems it has found have been uncovered by other Trent 900 operators Singapore Airlines and Lufthansa.

One line of inquiry centres on the Los Angeles-Melbourne run, a 15-hour-plus trip that sees the superjumbos leave fully laden at maximum take-off thrust instead of using lower power take-offs common to other routes. A theory is that extra stress placed on planes operating the route is different from out-of-factory expectations.

Joyce has vowed not to return the A380s to service until the problem has been resolved. While that may affect the airline's bottom line as it charters planes and recompenses passengers, it is better than the alternative.

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Loads of power up in the air
The Rolls-Royce Trent 900 that powers Qantas's four-engine Airbus A380 is cleared at take-off thrust of 80,000lb.

With a fan diameter of 116 inches (295cm), the Trent 900 is the largest engine built by Rolls-Royce.

It has new-generation swept titanium fan blades with a scimitar-shaped leading edge for lower noise and greater efficiency.

Rolls-Royce claims it has the lowest emissions of any large turbo-fan engine, measured per pound of thrust.

At take-off, the A380's four Trent 900s will deliver thrust equivalent to the power of more than 3500 family cars.

The engine's hollow, titanium fan blades are almost 10 feet (305cm) across and suck in more than 1.25 tonnes of air every second.

By the time the air leaves the nozzle at the back of the engine, it has been accelerated to a speed of nearly 1600km/h.

Temperatures in the engine core are half those on the surface of the sun.

The blades in the engine's high pressure system rotate at 12,500rpm, with tip speeds reaching 2000km/h.

At take-off, each of the the 116-inch fans operates at nearly 3000rpm, with tip speeds 1.5 times the speed of sound.

A Trent 900 has about 20,000 individual components.

Republication of article from: The Australian November 09, 2010

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